When offshore oil and gas wells are drilled and completed subsea, the production can flow through flow lines to either a pipline to shore, or to a nearby fixed structure, or to a floating production facility. For floating production facilities, it is usually feasible to run all the individual flow lines to the surface. If the floating production facility is a ship-shaped vessel, it is usually moored by a single-point mooring arrangement, whereby the vessel can weathervane around the mooring point. This enables the vessel to head into the waves and wind, and reduce the mooring loads. As a consequence, it is possible for the vessel to continuously rotate in the same direction around the mooring point. With more than one production flow line passing through the mooring point, it is necessary to have some form of swivel. Thus, the swivel must be capable of having many flow paths through it and the complete assembly must be able to make multi passes as it rotates. Usually, this is accomplished by having all the flow paths bundled together in a central core or shaft, and then having each path individually exit sideways into an annular ring that rotates around the core. Swivels like this already exist, but for relatively low pressures and for liquids.
To date, there are not very many floating production systems, and usually they are on small, low-pressure oil fields. The associated gas is often used for power generation and the surplus flared, i.e. wasted. In future, it is expected that floating production systems will be used on high-pressure fields (up to 10,000 psi) and that government regulations will not permit the flaring of gas. One option is to reinject the associated gas back into the reservoir, which means pressurizing the gas to a higher pressure than the reservoir. Thus there is a need for a high-pressure, multipass swivel that can handle gas as well as oil.
The critical area of a swivel is the sealing system. The higher the pressure, the more difficulty in maintaining a seal, and gas is even more difficult. The present invention addresses the problem of sealing.
The traditional method of sealing swivels is to use an elastomer or plastic material for the seal element, often with reinforcing. There are many shapes and configurations of the seal itself. One important criteria is the seal gap--the distance between the two parts being sealed. The larger the gap or the higher the pressure, the more the possibility of the seal being extruded into the gap. In general, the larger the gap, the larger the seal must be, and the material must be harder or stronger. As the seal size increases, so does the frictional resistance, which can be a significant problem with multipass swivels. Although it is possible to make a swivel with an extremely small seal gap, the gap will not be maintained as the assembly is pressurized with production fluid. The annular ring of the swivel will expand under pressure and the seal gap will increase. Adding more material to the annular ring will help the problem but it becomes impractical before satisfactory gap control is reached. The present invention provides an alternative means of controlling the seal gap. It uses the deflections of the outer (annular) ring in a way that compensates for radial expansion.